It's civilization annihilated by Sith Lord Darth Malak during the Jedi Civil War, the planet Taris is a symbol of hope for the Republic, and the latest planet revealed for BioWare's upcoming MMO.
You may remember Taris as the first planet you visit in Knights of the Old Republic after escaping from the Endar Spire. It wasn't a particularly nice planet, what with the gang wars and rakghoul, citizens mutated by a Sith bioweapon, but it was a home for some, until Darth Malak came along and destroyed everything.
Now it's 300 years later, and the Republic is trying to rebuild the post-apocalyptic swampland into something more habitable. The under city is still there, and the rakghouls are still there, but there is something new - hope.
Yes, that was far too cheesy, even for me. Visit the Star Wars: The Old Republic Holonet for more info, while I recover from that last sentence.